Killer tree-ants snare prey in gruesome traps

Amazonian ants rig up gruesome traps to snare prey before stinging them to death and carving them up to eat, a new study reveals.

Allomerus decemarticulatus is a tiny tree-dwelling ant which lives in the forests of the northern Amazon. Researchers examining the relationship between different ant species and their host plants noticed that this particular ant lived on only one plant - Hirtella physophora - and that they built galleries hanging under its stems.

Many ant species build these galleries as hideouts to act as sanctuaries between their nests and foraging areas. But the team, led by Jérôme Orivel at the University of Toulouse, France, spotted that A. decemarticulatus were using these galleries as traps for prey.

The traps are woven together using hairs stripped from the ants' host plant and reinforced with fungus, producing a platform with pitted holes. "The ants are always hiding just under the holes, waiting with their mandibles open. When an insect arrives they immediately grab the legs and antennae," says Orivel. This pulling immobilises the victim, stretching it out as though being tortured on a mediaeval rack.

Worker ants then clamber over their helpless prey, biting and stinging until the victim is paralysed or dead. The carcass is then chopped into small pieces while still on the rack or, more likely, carried back to the leaf pouch to be devoured. The surprise-attack traps are "like something out of Edgar Allan Poe", says Mike Kaspari, an ant expert at the University of Oklahoma, US.

By pulling off the plants' sharp stem hairs - which normally deter small herbivores - the ants are also cleverly deceiving their insect prey into resting on a comfortable "landing platform", bare of the hairs, he explains.

Fungus farmers

The collective creation of a trap to capture prey has not been reported before for ants, the researchers believe. Indeed, Orivel says that the only other insects known to do this are social spiders, and these use their own silk, not gathered material.

He also notes that Hirtella physophora plants without these ant colonies do not bear the fungus used to reinforce the traps, suggesting the ants farm the fungus.

The grisly traps enable the ants to capture large prey they could never otherwise eat and which provide a valuable source of nitrogen for their diet. The 2-millimetre-long ant species can ensnare insects over 3 centimetres in length and weighing up to 1000 times the weight of an individual ant. They can entrap grasshoppers, crickets or caterpillars, for example.

"Their ability to overcome their small size by engineering this solution is pretty impressive," Kaspari told New Scientist. "For me, one of the take-home messages is that being tiny is not a problem if you are numerous and organised."

Journal reference: Nature (vol 434, p 973)

If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.

Hordes of ants scuttle from their hidey-holes to grab hold of the large victim's legs and antennae, immobilising it (Image: Alain Dejean/Jerome Orivel)